In general, a rotary encoder for detecting a rotation angle of an object to be measured includes an optical scale having bright and dark optical patterns, a detecting element for detecting the optical patterns on the optical scale, and a calculator disposed at a subsequent stage of the detecting element, wherein the calculator detects a rotation angle of the optical scale that is coupled to a rotary shaft of a motor or the like.
As this type of rotary encoder, there are known an incremental system in which the calculator detects a rotation angle by accumulating pulse signals outputted from the detecting element, and an absolute system in which the calculator detects an absolute angle of the optical scale in accordance with the angularly unique optical patterns on the optical scale. In the incremental system, since the rotation angle is detected by an increment from an origin, it is thus necessary to perform an origin return operation when powering on the optical encoder. On the other hand, in the absolute system, accumulation of pulse signals is not required and thus it is unnecessary to perform an origin return operation when powering on the optical encoder. This allows quick restart from emergency stop or power failure.
There is further known a system of modulating optical patterns on an optical scale as a technique for detection of an absolute angle or an absolute position. For example, a linear scale measurement device according to Patent Document 1 includes an optical scale having transparent portions and opaque portions, a projector for irradiating the optical scale with light, and a light receiver for receiving light from the optical scale, wherein the line widths of the opaque portions on the optical scale are gradually varied from thinner lines to thicker lines so that an amount of transmitted light detected by the light receiver is sinusoidally varied.
A position detection device according to Patent Document 2 includes an optical scale having light shielding portions and light transmitting portions, a projector for irradiating the optical scale with light, and a light receiver for receiving light from the optical scale, wherein lengths of the light shielding portions on the optical scale are modulated so that an absolute amount of light transmitting the optical scale is monotonously decreased or increased.